Composite nitrided materials

ABSTRACT

Graded nitrided articles, surface modified in alloy composition wherein the surface zone consists of nitrided alloys consisting essentially of (A) one or more metals of the group columbium, tantalum, and vanadium; (B) titanium; and (C) one or both metals of the group molybdenum and tungsten. A minor portion of the nitrogen may be replaced by oxygen or boron. Nitrided materials prepared from homogeneous alloys are also included. The materials are characterized by excellent wear and abrasion resistance.

United States Patent 1191 Rausch et al.

[ 1 COMPOSITE NITRIDED MATERIALS [75] Inventors: JohnJ. Rausch, Antioch;Ray J.

Van Thyne, Oak Lawn, both of 111.

[73] Assignee: Surface Technology Corporation,

Stone Park, 111.

[22] Filed: Mar. 20, 1972 [21] Appl.No.: 236,213

Related U.S. Application Data [60] Division of Ser. No. 16,595, March 4,1970, Pat. No. 3,674,547; which is a continuation-in-part of Ser. .No.755,658, Aug. 27, 1968, Pat. No. 3,549,427.

52 us. c1 148/315, 29/1823, 29/195, 29/198, 148/203 51 1m. 01... C22C27/00, c220 29/00, C23C 11/14 [58] Field of Search 75/134, 135, 174,175.5,

Van Note 148/203 [111 3,802,933 45] Apr. 9, 1974 Berger et'al. 75/1773,161,503 12/1964 Lenning et a1..l 75/174 3,163,563 12/1964 Douglass etal..... 148/34 3,173,784 3/1965 wlodek et a1. 75/174 3,471,342 10/1969Wood 148/315 OTHER PUBLICATIONS Belgian Patents Report No. 10/69, 7:Metallurgy p. 1 Nos. 720398 84.720399.

1R 7187(l1l), IIT Research, ,lune l6, l967-Sept.15, 1967, pp. 51-55, 59,60, 65 and 67 Primary Examiner-Charles N. Lovell Attorney, Agent, orFirm-Albert Siegel [57] v ABSTRACT Graded nitrided articles, surfacemodified in alloy composition wherein the surface-zone consists ofnitrided alloys consisting essentially of (A) one or more metals of thegroup columbium, tantalum, and vanadium; (B) titanium; and (C) one orboth metals of the group molybdenum and tungsten. A minor portion of thenitrogen may be replaced by oxygen or boron. Nitrided materials preparedfrom homogeneous alloys are also included. The materials arecharacterized by excellent wear and abrasion resistance.

6 Claims, No Drawings I 1 COMPOSITE NITRIDED MATERIALS CROSS REFERENCETO RELATED APPLICATION This is a division of application Ser. No. 16,595filed March 4, 1970, now U.S. Pat. No. 3,674,547, which is in turn acontinuation-in-part of application, Ser. No. 755,658, entitled WEARRESISTANT MATERI- ALS" filed Aug. 27, 1968 now U.S. Pat. No. 3,549,427.

BACKGROUND OF THE INVENTION In our parent application, Ser. No. 755,658,referenced above, we have disclosed and claimed certain ni trided alloysconsisting essentially of v a. at least one metal of the groupcolumbium, tantalum and vanadium;

b. titanium; and

c. at least one metal of the group molybdenum and tungsten in certainpercentages by weight and compositional relationships as are therein setforth. Such nitrided materials are characterized by, among others,excellent wear and abrasion resistance and offer substantial utility ascutting tool materials.

In such parentapplication, we have noted that the desired alloys to benitrided may be formed as free standing thin sections or clad or byvarious means formed as a coating upon different substrates. Similarly,in such parent application, we have noted that a variety of nitridingtreatments may be employed to 'effectuate the desired results.

In the present application, we wish to elaborate upon the teachings ofsaid parent application. The compositions-hereof'which are nitrided orotherwise treated are the same as the alloy compositions which aredisclosed in our parent application.

Accordingly, our parent application, Se'r. No. 755,658 now U.S. Pat. No.3,549,427, in its entirety, is

Metallurgical Society Conference,Vol. 10, 1961, P.

In the present application, we wish to clearly point out thesignificance of alloying surface treatments or coatings or clad dingswith the present materials and surface treatments wherein nitriding isemployed as the major constituent along with relatively minor amounts ofoxygenand/or boron.

It should be noted that the alloys of the present invention may beemployed on another metal or alloy as a surface coating or cladding andwith the proper substrate selection, a highly ductile and/or essentiallyunreacted substrate can be obtained. For example, 00-

lumbium or tantalum are much lessreactive to nitrogen when used inconjuction with the alloys hereof and tungsten and molybdenum do notform stable nitrides at the nitriding temperatures employed. Sprayingand- [or fusing the-desired alloy onto the surface are included in thevarious coating methods available. A variety of direct depositionmethods may be employed or alternate layers could be deposited followedby a diffusion annealing treatment. I

As set out in our parent application in determining whether or not amaterial falls within the scope thereof, certain test criteria were usedas are set forth therein.

More particularly, following nitrided sample'preparaincorporated hereinby reference. We would note that a counterpart of such parentapplicationhas issued as Belgium Pat. No. 720,398. As will be evident,we herein provide additional features to said basic invention andcertain improvements thereof.

In our parent application, the temperatures are presented uncorrected.In the present application, temperatures are corrected. We used acorrection factor determined by using a tungsten-rhenium thermocouple inconjunction with the sightings of the optical pyrometer mentioned in theparent case. 7

Furthermore, wewould note that it is well known that titanium can benitrided to form a hard surface layer thereon but such material shows achipping propensity due to brittleness. In the practice of our invention, such brittleness is avoided by specific alloying as taught hereinprior to nitriding. Additionally, the alloying elements present intypical commercially available titanium alloys do not produce the sameimprovement and nitrided commercial titanium alloys show chippingsimilar to nitrided titanium.

The nitriding of titanium-rich alloys, -i.e., containing about 90percent titanium has been studied previously (for example, see E.Mitchell and P. J. Brotherton, J. Institute of Metals V01. V93 19.64) P.381 Others have investigated the nitriding of hafnium-base alloys (F.Holtz, et al., U.S. Air Force Report IR-718-7 (II) tion lathe turningtests were run thereon at surface speeds from 100 to 750 surface feetper minute (SFM) on A181 4340 steel having a hardness of around RockwellC, (Rc), 43 to 45. A feed rate of 0.005 in./rev. and depth of cut of0.050 in. were .used. A'standard negative rake tool holder was employedwith a 5 back rake and a 15 side cutting edge angle. Tool wear wasmeasured after removing a given amount of material.

The principal criterion in our parent application in 'inch-metal removalof the 4,340 steel at speeds of both and 750 SFM.

At 750 SFM our high performance, nitridedmaterials readily pass theinitial test of 2 cu. in. metal removal in about 1 minute. (We wouldnote that by SFM'is meant the linear rate at which the materialbeing'c'ut passes the cutter.) v

In some aspects of the present invention, such'test criteria of theparent application are inoperative. This is particularly true of thethin sections and surface zones considered herein. The nitrided alloysare the same but in some instancein, thin sections the test criteria ofthe parent case are'not met herein. However, the materials still offersubstantial wear and abrasion resistant properties.

In evaluating tools and tool materials, failure is often assumed tooccur when the wearland reaches 0.030 inch. With the materials of thisinvention, we selected a rather severe test we indicate those which aregood (i.e., pass the test), when at 750 SFM and 2 cu. in. re-' moval,there is a uniform wearland of less than 0.025 in. Furthermore, we wouldnote that although chipping is seen in some compositions upon testing at75C SFM the chipping propensity is aggravated at lower speeds and betterassessed at 100 SFM; The latter is one of the reasons for selecting bothspeeds.

Accordingly, a principal object of our invention is to provide certainnovel articles wherein the surface zone thereof is a nitrided alloyconsisting essentially of: (a)

at least one metal of the group columbium, tantalum and vanadium; (b)titanium; and (c) at least one metal nitrogen weight-pickup is replacedby oxygen and/or boron. I

These and other objects, features and advantages of our inventionwillbecome apparent to those skilled in this art from the followingdetailed disclosure thereof.

DESCRIPTION OF THE INVENTION geneous materials may be used for a widevariety of ap- An alloy of the composition, Cb-V40Ti-10Mo was readilyreduced to foil by rolling and coatings thereof were made on molybdenumby fusing this alloy Cb-l 8Til s'w alloy.

A 3 mil coating of Cb20V-40Ti-l0' Mowas also produced on molybdenum byfusing in argon. This was subsequently nitrided at 2,250F for one-halfhour resulting in a-nitrogen weight pickup of 1.6 mg per sq. cm. Themicrohardness at a depth of one-third mil from the surface was 1,680DPN. The nitriding temperature is sufficiently low that such alloysmaybe coated on a variety of substrate materials including ferrousalloys and successfully nitrided to produce a hard surface.

Much thinner coatings are readilyproduced by similar or otherprocedures. As the reactive alloy coating becomes thinner, the amount ofnitrogen pickup for surface hardening is reduced since the nitriding iscon-j centrated near the surface. Accordingly, in such thin sections thedepth of hardening is reduced. In relatively thin coatings, the weightpickup of .nitrogenmay be 0.1

to 1 mg persq. cm.- or less and in thicker coatings the pickup will beover 1 mg per sq. cm. of surface area.

In our copending, referenced parent application, we have shown that fornoncoated homogeneous alloy stock the amount of nitriding required forequivalent surface hardening is dependent upon sample thickness. As thethickness is decreased, the required nitriding temperature and weightpickup are reduced. We have observed a pronounced effect of specimenthickness, particularly at knife edges where the required nitrogenpickup is greatly reduced. Also, such coated or homoplications requiringwear and abrasion resistance where the requirement for surface hardnessor depth of hardening may be less than that required for metal cutting.Accordingly, in thin sections of homogeneous alloy material, similar tothin coatings of the alloys, the weight pickup of nitrogen may be 0.1 to1 mg per sq. cm.,

Another useful method for utilizing our nitrided materials involvescontrolled evaporation of titanium from the surface of an alloy(detitanizing). By this procedure,an alloy, for example, with a titaniumcontent greater than that determined by our compositional limitations,can be depleted in titanium content to bring the surface alloy contentwithin our prescribed ranges prior to nitriding. We have heated variousalloys containing the required metals of our invention in vacuo attemperatures below the melting point of the alloy. Titanium evaporationoccurred without any substantial change in geometry. Most importantly,this was accom- 20 plished without the occurrence of significant amountsof porosity. Electron microprobe analyses confirmed the significantchanges in weight that had been ob- I served. A specimen of Cb-45Ti-l0M0 vacuum treated at a pressure of 5: X 10 Torr at 2,85O,"F for 4 hoursshowed a decrease in titanium content and a corresponding increase incolumbium and molybdenum content.. The decrease intitanium contentextended to a considerable depth and in the outer 2 mils the decreasewas about 10 percent, Other vacuum treatments run at 2,95 0F for 6 hoursshowed even greater titanium loss.

'-A Cb-45Ti-2OW alloy vacuum treated at- 2,850F for 4 hours lost 33 mgfor a X if: X 43 inch specimen weighing l .9 gram, anda similar sizesample of Cb-SO- Ti-2OW vacuum treated at 2,950F for 6 hours lostSimilar detitanizing effects were shown for'Ta-Ti- 750 and lQO SFM. Allsuch'vacuum treated materials show high surface hardness. It will, ofcourse, be appreciated that such surface evaporation techniques can'beapplied to alloys that are alreacly within our prescribed 1 compositionranges to'effect desirable structural and property changes. I

The cutting performance of such Cb-4OTi-IOM0 vacuum treated at 2,850Ffor 4 hours prior to nitriding at 3,250F was better than the same alloywhen nitrided without prior detitanizing. It should be noted thatannealing per se, that is, annealing under conditions where significantevaporation does not occur, has an effect on the microstructuralmorphology. Such morphology effects due to annealing, which result ingreater regularity of structure may produce improvements for certainuses, but the compositional effect due to treatment in vacuo is of valueby itself.

Since our nitrided materials present as a homogeneous material or as acoated article are in a thermodynamically metastable condition,thoseskilled in'the art will realize that a variety of heat treatments,including multiple and sequential treatments, can be used to modify thereaction structure and resulting properties whether performed as partthe over-all nitriding reaction or as separate treatments. Improvementin cutting properties has been noted by nitriding at lower temperaturesfor longer times and by nitriding at'lower temperatures followed bynitriding at higher temperatures. However, the required weight pickupfor cutting at 750 SFM is similar to the amount of nitriding necessarywith a simple 2-hour nitriding treatment. The treatments have includedtypical nitriding followed by aging at lower temperatures in argon ornitrogen. We have also nitrided at higher temperatures (and longertimes) that normally would produce some embrittlement and thensubsequently annealed in inert gas or at various partial pressures-ofnitrogen as a tempering or drawing operation to improve toughness. Thisduplex treatment results in a greater reaction depth with thehardnesstoughness relationship controlled by the tempering temperatureand time.

Such treatments can be employed to modify the properties of our nitridedmaterials to produce various combinations of hardness and toughness. Therequired annealing treatmentis dependent upon the material usage, alloycomposition and degree of prior nitriding.

The influence of annealing under various conditions for a variety ofnitrided materials may be seen from the data presented in Table I.

750 and 100 SFM, improvement was achieved by nitriding at 3,250F for 2hours followed by annealing in argon at 3,250F for 1 hour. Also, goodcombined performance at 750 and 100 SFM was shown for Cb-Ti- 20Wnitrided at 3,550F for 2 hours and annealed at 3,550F for 1 hour.Annealing at 3,250F for 1 hour did not produce any significantimprovement and anneal ing for 4 hours at 3,550F resulted in failure incutting at 750 SFM. Thus, one should use due care in anneal-' ingconditions.

in most of our materials, the hardness (and nitride content) grades andlessens as one moves from the surface inwardly. However, we would notethat in some cases such grading extends from a plateau or from a peakhardness slightly below the surface and grades in wardly therefrom. Suchmaterials can be effective cutting tools .or abrasion resistantarticles.

We have also nitrided materials directly in an environment'sufficientlylow in nitrogen potential that the effect 'is noted. Nitriding inflowing A-0.l%N produces reduced nitrogen pick-up compared to 100percent nitrogen. Another method involves sealing the I furnace with ameasured amount of nitrogen and allowing the nitrogen content to bereduced during treat TABLE I Nitriding Argon Microhardness (DPN) atAlloy Treatment Treatment Depth (mils) Compositon F Hrs F Hrs 0.5 l 2 48 Cbl7Ti-20W v 3450 2 none 2570 .2090 1890 11 906 do do do 3450 l 12201017 1040 857 do do do 3450 l" 2l90 I420 I250 835 765 do do do 3250 23060 2600 2570 2160 985 Ta20Ti-l0Mo 3550 2 none 2060. 1675 1480 1110 dodo do 3250 l 1690 1175 1250 946 do do do 3250 4 1790 l 160 996 1060Argon-0.1 percent nitrogen atmosphere.

The alloy Cb-l7Ti-2OW, nitrided at 3,450F for 2 hours shows substantialsoftening when subsequently annealed in argon for 2 hours at this sametemperature. If the annealing is carried out in an atmosphere of A-0.l%Nit may be noted that only a moderate decrease in hardness occurs and thematerial grades uniformly in a manner similar to the nitrided condition.If

argon on reducing the uniformhardness gradient for the nitridedTa-20Ti-10Mo all-0y may also be seen from the above data. We have foundthat nitrided alloys containing higher amounts of tungsten or molybdenumsoften readily when annealed in argon. To control this softening, thatis, avoiding the formation of a surface-layer that is too soft to cutthe hardenedsteel at 750 SFM, we have found regulation of the nitrogencontent of the atmosphere to be a useful parameter. It should be notedthat the A0.l%N atmosphere will harden unnitrided or moderately nitridedalloys but results in softening when used with the highly nitridedalloys in the examples above. A X X Vs inch specimen of Cb-30Ti-20Wreacted in nitrogen at 3,250F

for 2 hours, cuts well at 750 SFM. When subsequently treated in A0.1%Nfor 2 hours, this material contint es to nitride as evidenced by afurther 8 mg. pick-up.

ment as a result of the specimens absorbing the availthis manner cutwell at both 750 and SFM. The

alloy Cb8 0Ti-l0Mo falling outside our-invention, was nitrided in A-0.1%N for 2 hours at 3,050F. Similar to treating in nitrogen, the resultwas a thick continuous 3 mil nitride-surface layer and such materialfails immediately in testing at 750 SFM. These various alternatenitriding treatments may be applied to thematerials of our inventionwhether used as'a homogeneous alloy or as a coated or surface modifiedmaterial. In all of the nitriding treatments and particularly for thoseinvolving reduced nitrogen potential, the effect of .the varyingstabilities of the metal nitrides must be considered since this can alsocontribute to-surface compositional effects.

Surface alloying techniques are also useful for the preparation of thealloys'to be nitrided to produce the materials of our invention. Cb-lOMowas titanized at 2,950F for 3 hours in vacuo by holding in a pack offine titanium-sponge which causes diffusion of titanium into thesurface. This treatment resulted in 216 mil titanized layer which uponnitriding for 2 hours at 325091 yielded a graded reaction zone similarto Cb.Ti-Momaterials. This contrasts with the 4. mil continuous nitridelayer formed on Cb-lOMo without the prior titanizing treatment whichexhibits cracking of the continuous nitride layer. 5

. than the total of (That is, the concentration of'columbium to totalcolumbium, tantalum and vanadium). Similarly,

When, in the present alloy systems, more than 1 metal of the groupcolumbium, tantalum and vanadium is present the maximum total content,-in terms of weight per cent of such metals must be equal 85 (RatioA) 88(Ratio B) 90(Ratio C) and the minimum-content thereof when tungsten and-/or molybdenum are present must be equal to or greater than thetotal ofI [(Ratio A) (Ratio 13)) [10 (Ratio E) 25 (Ratio D) (Ratio C)Furthermore, when there is more than 1 metal of the groupcolumbium,'ta'ntalum and vanadium present the maximum amount of titaniumpermitted in the alloy system is equal to orless than the amountdetermined by the. formula 45 (Ratio A Ratio. 0 35 (Ratio B) and theratio of the content of such metals to-the titanium must be greaterthanthe ratio determined, by

Ratio A ono 3 +0.66 (Ratio cm Additionally, when both tungsten andmolybdenum are present the maximum amount thereof is determined by theformula v I 60 (Ratio A Ratio 0 (Ratio D) U '50 (Ratio B) (Ratio 1)) 80(Ratio E):

We would further note that when colurnbium alone is-used of Group Ametals and both molybdenum and tungsten are'present the minimum amountof colu m bium required, is determined by the formula l-O (Ratio E)(Ratio D) The minimum titanium is 1 percent and the minimum tungstenand/or molybdenum is 2. percent.

f Up' to 3 percent of the titanium content may be replaced by zirconium.

Ta-10W was also titanizedwith the following procedure: vacuum packtitanized 2,950F 6 hours; an-

nealed in argon 2,950F 2 hours plus 3,0 50F 2 hours; nitrided 2,850F 2hours. Such treatment proto or less' ,duceda 6 mil titaniz ed diffusionzone. Structural grading is shown in the microhardness traverse datapresented below:

Microhardness (DPN) at depth (mils) 0.5 2 4 8 A strip specimen 72 milsthick was prepared using the same titanizing and nitriding proceduresand was subsequently bent Cracking of the hard nitrided case occurred onthe tension (outer) side. The adherency of I the hard nitrided 6 milzone was shown by the fact that none of it spalled from the Tal 0Wsubstrate which was intact.

Another surface alloying procedure involved the. combined titanizing anvanadizing of molybdenum or tungsten. This can be accomplished by vacuumpack treatment since titanium and vanadium have similar vapor pressures.Such treatment of molybdenum or tungsten a't 2,950F for.3.- hours yieldsa thinner diffusion zone than that observed for the titanizing ofCb-lSMo. The depth of the diffusion zone was about I 1 /2 mil withmolybdenum and less with tungsten. After nitriding at 3,250"F for 2hours-the microhardness of the molybdenum sample was 1000,605, and 190DPN at 0.5, 1., and 2 mils, respectively.

Use of surface alloying or coating techniques can enhance the utility ofpowder processing of the alloys prior to nitriding in a number of ways.For example, a powder processed alloy of Cb-Mo could be formed and thentitanized or a porous molybdenum or tungsten presinteredcompact could beinfiltrated by coating methods. Theseand other techniques can (1) lowersintering temperatures,'(2) enhance filling of pores, and (3) reduceshrinkage as compared to makinga homogeneous powder part.

We have modified our nitrided material by combining nitriding withoxidizing or boronizing. However, the amount of reaction with such otherhardening agents must be limited, a majority of the weight pick-up isdue tonitriding, and these are essentially nitrided materials. Thealloys may be preoxidi'zed at a temperture where'little reaction wouldoccur with nitrogen alone and then subsequently nitrided. Also, thealloys may be i reacted with a combined oxidizing and nitridingenvironment although the relative oxidizing potential must be low sincefor example in air the alloys will preferentially oxidize rather thannitride. A sample of Cb-30'li- -20W was nitrided at 3,250F for 2 hoursand subsequently boronized at 2,650F for 4 hours. The structural.features of such a materialarevery similar to the alloy only nitrided;the hardness grades inwardly and of i the total weight pick-up overpercent due to nitriding. A smooth surface layer about 0.4 mil thickforms due to the boronizing treatment that is harder than the nitridedsurface.

For comparison, the Cb-30Ti-2OW alloy nitrided at 3250F for 2 hoursexhibits a microhardness of 2680 I DPN at a distance of one-third milfrom the surface. After the subsequent boronizing treatment, thehardness was 4,550 DPN at the same depth. This duplex treated materialpasses our test at 750 and SFM but the chipping propensity is increased.Up to 25 percent of the nitrogen pick-up byweight may be replaced byoxygen and/or boron.

Although the alloys receptive to nitriding can be pro duced by coatingor surface alloying techniques, many uses involve the forming andmachining of a homogeneous alloy or a coated article. One of theadvantages in utility of these materials is our ability to form themetallic alloys by cold or hot working and/or to machine (or hone) toshape in the relatively soft condition prior to final nitriding. Onlyminimal distortion occurs during nitriding and replication of thestarting shape and surface finish is excellent. The final surface isreproducible and is controlled by original surface condition, alloycomposition, and nitriding treatment. For some applications, the utilitywould be enhanced by lapping, polishing, or other finishing operationsafter nitriding. The nitrided surface is quite hard but only a smallamount of material removal is required to produce a highly finishedsurface.

One of the nitrided effects that we have noticed is an accentuation ofsharp edges. Similar to the established technology for aluminum oxideceramic insert tools, we have blunted sharp cutting edges prior tonitriding. This has been accomplished by simple tumbling prior tonitriding or by finishing subsequent to nitriding. High speed cuttingperformance will not be degraded if such edge preparation is limited.The nitrided material can be used as a mechanically locked insert or itcan .be bonded or joined by brazing, for example, to a substrate.

We have also observed the excellent corrosion resistance of both thealloys and the nitrided alloys in strong acids, and these materialscould effectively be employed for applications requiring both corrosionand abrasion resistance. Both the alloys and the nitrided alloys possessgood structural strength. Thus, the materials can be employed forapplications involving wear resistance and structural properties(hardness, strength, stiffness, toughness) at room and elevatedtemperatures. Other useful properties of the nitrided materials includegood electrical and thermal conductivity, high melting temperature, andthermal shock resistance.

The excellent cutting pr operties and wear resistance of the nitridedmaterials can be effectively employed with the other useful propertiesof the alloys and nitrided materials to produce a wide range ofproducts. Some of these are: single point cutting tools, multiple pointcutting tools (including rotary burrs, files, routers and saws), drills,taps, punches, dies for extrusion, drawing, and other formingoperations, armor, gun barrel liners, impeller of fan blades, EDP(Electrical Discharge Machining) electrodes, spinnerets, guides (thread,wire, and other), knives, razor blades, scrapers, slitters, shears,forming rolls, grinding media, pulverizing hammers and rolls, capstans,needles, gages (thread, plug, and ring), bearings and bushings, pivots,nozzles, cylinder liners, tire studs, pump parts, mechanical seals suchas rotary seals and valve components, engine components, brake plates,screens, feed screws, sprockets and chains, specialized electricalcontacts, fluid protection tubes, crucibles, molds and casting dies, anda variety of parts used in corrosion-abrasion environments in thepaper-making or petrochemical industries, for example.

It will be understood that various modifications and variations may beaffected without departing from the spirit or scope of the novelconcepts of our invention.

We claim as our invention:

substrate member; a graded nitrided ternary or higher alloyed surfacezone on said substrate member, which surface zone consists of at leastone metal selected from each of Groups A, B, and C whereinGroup Aconsists of columbium, tantalum and vanadium; Group B is titanium andGroup C consists of molybdenum and tungsten and wherein:

a. the nitrogen pickup of said surface zone is at least 0.1 milligramper square centimeter of surface area and in said zone;

b. when only columbium and molybdenum are present with titanium therange for the columbium content is from about 20 percent to percent;

c. when only columbium and tungsten are present with titanium the rangefor the columbium content is from about 10 percent to 85 percent;

(1. when only columbium, molybdenum and tungsten are present withtitanium the minimum amount of columbium required is determined by theformula.

10 (Ratio E) 20 (Ratio 1) and the maximum content of columbium is about85 percent;v

e. when only tantalum and molybdenum are present with titanium the rangefor the tantalum content is from about 25 percent to 88 percent;

f. when only tantalum and tungsten are present with titanium the rangefor the tantalum content is about 10 percent to 88 percent;

g. when only tantalum, molybdenum and tungsten are present withtitanium. the minimum amount of tantalum required is determined by theformula 10 (Ratio E 25 (Ratio D) and the maximum content of tantalum isabout 88 percent;

h. when only vanadium and a metal selected from the group consisting ofmolybdenum and tungsten and combinations thereof are presentwithtitanium the range for the vanadium content is about 15 percent to90 percent;

i. when more than one metal of the group columbium, tantalum andvanadium are present with only molybdenum and titanium the minimum totalcontent of the metals columbium, tantalum and vanadium must be at leastequal to the amount of 20 (Ratio A) 25 (Ratio B) 15 (Ratio C);

j. when more than one metal of the group columbium, tantalum andvanadium are present with only tungsten and titanium, the minimum totalcontent of the metals columbium, tantalum and vanadium must be atleastequal to the amount of l0 (Ratio A) 10 (Ratio B) 15 (Ratio C);

k. when more than one metal of the group columbium, tantalum andvanadium are present with molybdenum, tungsten and titanium, the minimumtotal content of the metals columbium, tantalum and vanadium must be atleast equal to the amount [(Ratio A) (Ratio 8)] [l0 (Ratio E) 25 (Ratio0)] 15 (Ratio C);

l. when more than one metal of the group columbium, tantalum andvanadium are present the maximum total content thereof must be equal toor less than 85 (Ratio 'A) 88 (Ratio B) 90 (Ratio C);

m. when titanium is present with only columbium and 0. when titanium ispresent only with vanadium and a metal selected from the groupmolybdenum and tungsten and combinations thereof, the titanium 45 (RatioA Ratio C) 35 (Ratio B) and the ratio of the content of the'metalscolumbium, tantalum and vanadium to titanium must be equal to or greaterthan the ratio of (Ratio A) (Ratio B) 0.66 (Ratio cm and the minimumtitanium content is 1 percent,

q. when only molybdenum, titanium and a metal selected from groupcolumbium and vanadium and combinations thereof are present, the rangefor molybdenum content is from about 2 percent to 60 percent; v i v r.when only molybdenum, titanium .and-tantalum'are present the range ofthe molybdenum content is from about 2 percent to 50 percent;

s. when only tungsten, titanium and a metal selected from the groupcolumbium, tantalum and vanadium and combinations thereof are presentthe range for tungsten content is from about 2 percent to percent;

t. when molybdenum, tungsten, titanium and a metal selected from thegroup columbium, tantalum, va nadium and combinations thereof arepresent the maximum total content of molybdenum and tungsten must beequal to or less than 6O (Ratio A Ratio C) (Ratio D) 50 (Ratio B) (RatioD) 80 (Ratio E) and the minimum content of molybdenum and tungsten is 2percent; u. and wherein in the foregoing Ratio A Cb/(Cb Ta V) Ratio BTa/(Cb Ta V) Ratio C V/(Cb Ta V) Ratio D Mo/(Mo W) Ratio E W/(Mo w) 2.The article as defined in claim 1 wherein in said surface zone up to 3percent of the titanium content thereof is replaced by zirconium.

3. The article as defined in claim 1 wherein the surface zone thereof issurface alloyed to the desired composition.

4. The article .as defined in claim 1 wherein the graded, nitridedportion of the surface zone thereof is depleted in titanium content tothe desired composition.

5. The article as defined in claim 1 wherein said surface zone comprisesa discrete coating on said substrate. 1

6. The article as defined in claim I wherein up to 25 percent of thenitrogen weight pick-up is replaced by a material selected from thegroup consisting of oxygen and boron and mixtures thereof.

2. The article as defined in claim 1 wherein in said surface zone up to3 percent of the titanium content thereof is replaced by zirconium. 3.The article as defined in claim 1 wherein the surface zone thereof issurface alloyed to the desired composition.
 4. The article as defined inclaim 1 wherein the graded, nitrided portion of the surface zone thereofis depleted in titanium content to the desired composition.
 5. Thearticle as defined in claim 1 wherein said surface zone comprises adiscrete coating on said substrate.
 6. The article as defined in claim 1wherein up to 25 percent of the nitrogen weight pick-up is replaced by amaterial selected from the group consisting of oxygen and boron andmixtures thereof.